1. Field of the Invention
The present invention relates to an image signal processing apparatus, and more particularly to an image signal processing apparatus capable of detecting a dropout of an image signal, compensating for the drop-out with high fidelity, and displaying information on the drop-out.
2. Related Background Art
Referring to FIG. 2, in a prior art drop-out compensation circuit for processing analog image signals without using a memory, an FM modulated analog image signal is applied to an input terminal 2-1 and the envelope level thereof is detected by an envelope detector 2-2. The envelope is applied to a comparator 2-3 which generates a drop-out detection pulse DOC when the envelope level reaches a value smaller than a predetermined one, and supplies the drop-out detection pulse DOC to an analog switch 2-7. The analog switch 2-7, which is supplied with a present analog image signal and that delayed by 1H (horizontal period) by a 1H delay line, performs image processing by compensating the drop out of an analog image signal with an analog image signal before 1H period, upon reception of the drop-out detection pulse DOC. An apparatus is known which demodulates the processed signal by a demodulator 2-5 and converts the demodulated signal into a digital signal to store it in an image memory 2-8.
With the above construction, the processed image signal may often have distorted portions at the start and end of the drop-out when the analog switch 4 is operated. Further, it is very difficult to adjust the levels of image signals passing through the 1H delay line and those not passing through it.
Furthermore, since the lack of an image signal is simply replaced with an image signal before period 1H, high quality compensation cannot be expected when a drop-out occurs at the location where the lacked image signal has no substantial correlation with that before period 1H.
Still further, since the lack of an image signal is simply replaced with an image signal before period 1H, compensation is almost impossible when a drop-out occurs during consecutive several horizontal periods. However, the user cannot be notified of such effect. In addition, the range of a drop-out cannot be notified beforehand.